U.S. patent application number 13/223276 was filed with the patent office on 2012-03-08 for navigation and sample processing using an ion source containing both low-mass and high-mass species.
This patent application is currently assigned to FEI COMPANY. Invention is credited to Chad Rue.
Application Number | 20120056088 13/223276 |
Document ID | / |
Family ID | 45769990 |
Filed Date | 2012-03-08 |
United States Patent
Application |
20120056088 |
Kind Code |
A1 |
Rue; Chad |
March 8, 2012 |
Navigation and Sample Processing Using an Ion Source Containing
both Low-Mass and High-Mass Species
Abstract
An improved method and apparatus for imaging and milling a
substrate using a FIB system. Preferred embodiments of the present
invention use a mixture of light and heavy ions, focused to the
same focal point by the same beam optics, to simultaneously mill
the sample surface (primarily with the heavy ions) while the light
ions penetrate deeper into the sample to allow the generation of
images of subsurface features. Among other uses, preferred
embodiments of the present invention provide improved methods of
navigation and sample processing that can be used for various
circuit edit applications, such as backside circuit edit.
Inventors: |
Rue; Chad; (Portland,
OR) |
Assignee: |
FEI COMPANY
Hillsboro
OR
|
Family ID: |
45769990 |
Appl. No.: |
13/223276 |
Filed: |
August 31, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61378643 |
Aug 31, 2010 |
|
|
|
Current U.S.
Class: |
250/307 ;
250/306; 250/492.21 |
Current CPC
Class: |
H01J 37/317 20130101;
H01J 37/304 20130101; H01J 2237/0805 20130101; H01J 2237/30466
20130101; H01J 2237/31749 20130101; H01J 37/3005 20130101; H01J
2237/0825 20130101; H01J 2237/30472 20130101; H01J 37/3056
20130101 |
Class at
Publication: |
250/307 ;
250/306; 250/492.21 |
International
Class: |
G21K 5/04 20060101
G21K005/04; H01J 37/26 20060101 H01J037/26 |
Claims
1. An apparatus for high accuracy beam placement and navigation to
a feature of interest on a sample, comprising: a source of ions,
the source producing ions of more than one elemental species; and a
particle beam column for producing a coaxial mixed beam of ions
including ions of more than one elemental species and for focusing
the mixed beam to a focal point at or near the sample.
2. The apparatus of claim 1 in which the source of ions produces a
mixture of ions of a lighter elemental species having a mass of
<20 amu and a heavier elemental species having a mass of >28
amu.
3. The apparatus of claim 1 in which the source of ions produces a
mixture of ions of a lighter elemental species and ions of a
heavier elemental species, with the heavier elemental species
having an atomic mass that is at least double the atomic mass of
the lighter elemental species.
4. (canceled)
5. (canceled)
6. The apparatus of claim 1 in which the source of ions produces a
mixture of ions comprising ions of a lighter elemental species and
ions of a heavier elemental species, with the heavier elemental
species having an atomic mass that is at least 40 amu greater than
the atomic mass of the lighter elemental species.
7. The apparatus of claim 1 in which the source of ions produces a
mixture of ions comprising ions of a lighter elemental species and
ions of a heavier elemental species, the lighter elemental species
having an atomic mass that is low enough that the ions of the
lighter elemental species will penetrate the sample surface to a
depth of >120 nm when the mixed beam is focused onto the surface
and the heavier elemental species having an atomic mass that is
high enough that the ions of the heavier elemental species will
rapidly remove sample material by sputtering when the mixed beam is
focused onto the surface.
8. (canceled)
9. (canceled)
10. (canceled)
11. The apparatus of claim 2 in which the source of ions is a
plasma source, said plasma source including multiple gas sources
for delivering multiple gases at the same time to produce ions of
more than one elemental specie and in which the amounts of the
multiple gases delivered to the plasma source can be adjusted to
change the ratio of ions produced between the more than one
elemental species.
12. The apparatus of claim 1 in which the more than one elemental
species of ions in the mixed beam process and image the sample
simultaneously.
13. The apparatus of claim 2 in which, in operation, the ions of
the lighter elemental species can be used to produce a subsurface
image of the sample to a depth of >80 nm.
14. The apparatus of claim 13 in which, in operation, the ions of
the heavier elemental species can be used to rapidly remove sample
material.
15. The apparatus of claim 14 in which, in operation, the
subsurface image is used to control the material removal.
16. The apparatus of claim 14 in which, in operation, the ratio of
light to heavier elemental species in the mixed beam can be altered
to fine-tune the imaging and/or etching of the sample.
17. The apparatus of claim 14 further comprising a
computer-readable memory storing computer instructions, the
instructions including a program for controlling the apparatus to
carry out the steps of: detecting secondary electrons resulting
from the impact of ions of the lighter ion species on the sample to
form an image of a subsurface feature of the sample; using the
location of the imaged subsurface feature to direct the ion beam
toward the sample; and processing the sample using the ions of the
heavier ion species to mill the sample surface.
18. The apparatus of claim 1 in which the source of ions is an
alloy liquid metal ion source.
19. The apparatus of claim 18 in which the alloy is AuSiBe, AuSi,
or AsPdB.
20. The apparatus of claim 18 in which the alloy liquid metal ion
source produces one elemental species of ion having a lower mass
and another ion species having a higher mass.
21. The apparatus of claim 20 in which the type of alloy liquid
metal ion source used is determined by the ratio of lighter to
heavier ion species produced by a given alloy liquid metal ion
source.
22. The apparatus of claim 20 further comprising a mass filter for
filtering between the lower and higher mass ions so that the beam
comprises only ions of one of either the lighter or heavier ions,
and a computer-readable memory storing computer instructions, the
instructions including a program for controlling the apparatus for
causing rapid switching back and forth between the at least first
and second species of ions at a selected frequency.
23. The apparatus of claim 22 in which lighter ions are for
subsurface imaging of the sample and in which the heavier ions are
for processing the sample.
24. The apparatus of claim 23 in which the length of time that each
of the lighter or heavier ions is filtered by the mass filter is
adjusted to favor subsurface imaging rather than sample processing
or to favor sample processing rather than subsurface imaging.
25. The apparatus of claim 23 further comprising a
computer-readable memory storing computer instructions, the
instructions including a program for controlling the apparatus to
carry out the steps of: detecting secondary electrons resulting
from the impact of ions of the lighter ion species on the sample to
form an image of a subsurface feature of the sample; and using the
location of the imaged subsurface feature to direct the ion beam
toward the sample; and processing the sample using the ions of the
heavier ion species to mill the sample surface.
26. The apparatus of claim 18 in which the source of ions is an
alloy liquid metal ion source producing more than two different
elemental species of ions and further comprising a mass filter used
to filter out at least one elemental species of ions but allow a
mixed beam of two different elemental species of ions to be focused
onto the sample.
27. An apparatus for accurately locating a feature of interest on a
sample, comprising: a particle beam column for producing a beam of
charged particles along a beam axis to image and/or mill the
sample; a source of charged particles from which the particle beam
is composed, wherein the source of particles produces at least a
first and a second species of ions, said first and second species
having different elemental compositions and different atomic mass;
a mass filter for filtering between the at least first and second
species of ions so that the charged particle beam comprises only
ions of one of the first and second species of ions, and a
computer-readable memory storing computer instructions, the
instructions including a program for controlling the apparatus for
causing rapid switching back and forth between the at least first
and second species of ions at a selected frequency.
28. The apparatus of claim 27 in which the frequency for switch
between species of ions is at least every tenth of a second.
29. The apparatus of claim 27 in which the switch between species
of ions occurs at a high enough frequency for the imaging and
processing to appear to an operator to be simultaneous.
30. The apparatus of claim 27 in which switching back and forth
between the at least first and second species of ions at a selected
frequency is accomplished by applying a periodic function to the
mass filter voltage with a desired frequency.
31. The apparatus of claim 30 in which the frequency can be
adjusted during operation of the beam.
32. The apparatus of claim 28 in which first species of ions is a
lighter species of ions for subsurface imaging of the sample and in
which the second species of ions is a heavier species of ions for
processing the sample.
33. The apparatus of claim 32 in which the length of time that each
of the first and second species of ions is filtered by the mass
filter is adjusted to favor subsurface imaging rather than sample
processing or to favor sample processing rather than subsurface
imaging.
34. A method for processing a sample having subsurface features,
the method comprising: providing particle beam system including an
ion source that produces at least two species of ions and an ion
beam column for producing a beam of ions, the beam including the at
least two species of ions, and the at least two species of ions
having different elemental compositions and comprising a lighter
ion species having a lower atomic mass and a heavier ion species
having a higher atomic mass; directing the ion beam toward the
sample to image and process the sample; detecting secondary
electrons resulting from the impact of ions of the lighter ion
species on the sample to form an image of a subsurface feature of
the sample; using the location of the imaged subsurface feature to
direct the ion beam toward the sample; and processing the sample
using the ions of the heavier ion species to mill the sample
surface.
35. The method of claim 34 further comprising altering the
concentration of at least one of the at least two species of ions
in the beam to fine-tune the imaging and/or processing of the
sample.
36. The method of claim 35 in which altering the concentration of
at least one of the at least two species of ions occurs while the
sample is being processed and/or imaged.
37. The method of claim 34 further comprising using the image of
the subsurface feature to determine when to stop milling the sample
using the beam.
38. The method of claim 34 in which using the location of the
imaged subsurface feature to direct the ion beam toward the sample
comprises: superimposing a graphical representation of CAD design
data over the subsurface image and performing a registration of the
image to the CAD design data; navigating the particle beam system
to a target location using known coordinates from the CAD design
data; and re-imaging the subsurface feature to locate the target
and navigate the beam to the target location.
39. The method of claim 35 further comprising: providing a greater
concentration of lighter ions relative to heavier ions when the
beam is used for imaging subsurface images for navigation purposes,
but once the subsurface feature has been located, increasing the
concentration of heavier ions in the beam for faster sample
processing.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority from U.S. Prov. Pat. App.
No. 61/378,643, filed Aug. 31, 2010, which is hereby incorporated
by reference.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to stage navigation and beam
placement in particle beam systems and, in particular, to high
accuracy local area navigation to a site of interest on a sample
surface and mill end-pointing using a focused ion beam.
BACKGROUND OF THE INVENTION
[0003] Modern integrated circuits (ICs) are composed of multiple
layers of conductors and substrate materials, such as insulators
and semiconductors. Inspecting and editing a circuit or other
hidden interior feature in an IC requires navigating to the target
area and milling through one or more of the multiple layers of
substrate material. Circuit Edit (CE) reduces IC development costs
by reducing the number of mask sets that are required during the
design-debug phase, and speeds overall time-to-market.
[0004] Most CE activities today are performed with Focused Ion Beam
(FIB) systems, which are commonly used to mill away a substrate
material to expose hidden features and also deposit materials with
high precision. These capabilities can be used to cut and connect
circuitry within a device, as well as to create probe points for
electrical test. Applications include validating design changes,
debugging and optimizing devices in production, and prototyping new
devices without costly and time-consuming mask set fabrication.
[0005] Typically material removal in FIB systems is accomplished by
using beams of relatively large ions to physically sputter away the
substrate material. Most FIB systems use gallium ions produced by a
Liquid Metal Ion Source (LMIS) because such sources are easy to
fabricate, operate at room temperature, and are reliable, long
lived, and stable. Ion sources using indium are also known.
[0006] In LMIS systems, it is also known to use alloy sources
comprising metal alloys of two or more different elements. Prior
art alloy sources are typically equipped with mass filters so that
the desired ion species can be selected. Alloy sources are often
used because the desired ion species alone would be unsuitable for
use in a LMIS (for example when the elemental species has a too
high melting point) but the properties of the alloy are more
favorable. Alloy sources have also been used to switch between two
desired ion species for implantation, such as using an alloy source
producing beryllium and silicon ions to implant p-layer and n-layer
structures, respectively, on a gallium arsenide substrate.
[0007] Plasma ion sources have also been used to form ion beams.
The magnetically enhanced, inductively coupled plasma ion source
described in U.S. Pat. App. Pub. No. 2005/0183667 for a
"Magnetically enhanced, inductively coupled plasma source for a
focused ion beam system" can be used to produce a finely focused
beam with a relatively large beam current that can be used for CE
applications.
[0008] Although FIB systems can also be used to generate a sample
image while milling in order to monitor the milling process, the
image is typically restricted to the very top surface of the
sample. This causes problems for CE applications because many
modern ICs do not include visible surface features to serve as
reference points for navigation. This is especially true for
backside editing, which is becoming increasingly common for CE.
Instead of trying to mill through many layers of dense circuitry
from the front, operators turn the device over and mill through the
substrate silicon to access target areas from the back.
[0009] FIG. 1 shows a schematic representation of a typical prior
art backside IC device 10. As shown in FIG. 1, a solid layer of
silicon 12 typically covers the backside of the circuitry.
Underneath the silicon layer, the IC device shown in FIG. 1
includes an active region 14 and a number of deeper metal layers M1
through M5, with each layer including metal lines 16 and vias 18
surrounded by a dielectric material 20. FIG. 2 shows a schematic
representation of the backside IC device of FIG. 1 after a wedge
polish, which is an angled polish that exposes multiple layers at
once. In the schematic view of FIG. 2, it can be seen that the
wedge polish has removed all of the silicon and active layers and
exposed portions of metal layers M2 and M3. In a top-down view of a
sample such as the one shown in FIG. 2, as the sample is viewed
from left to right, in area 22 a via and portions of the dielectric
from layer M2 would be visible; in area 24 a portion of the M2
metal line would be visible; in area 26 portions of vias surrounded
by dielectric would be visible; and finally in area 28 the metal
line of layer M3 would be visible.
[0010] A wedge polish as shown in FIG. 2, while a convenient way of
looking at multiple layers at once, cannot be used for actual CE
because the IC device is destroyed. For CE of features hidden
beneath a sample surface, such as found in backside edits on bulk
silicon samples, it is typically necessary to precisely determine
the location of a desired buried feature and then to mill away
substrate material in order to expose that feature. Unfortunately,
it can be very difficult to locate such hidden features precisely.
Even when the beam is positioned correctly, it is often difficult
to expose the features without damaging the features with the ion
beam. Once the features are visible in the FIB image, some degree
of damage has already taken place. In other words, when using FIB
imaging to determine when a feature is exposed and milling should
be stopped, often referred to as end-pointing, the feature can be
damaged or even destroyed before the milling can be stopped.
Moreover, in order to find reference points in an image to
determine where on the circuit the feature of interest is located,
it is sometimes necessary to expose by trial and error a relatively
large area, potentially damaging each area that is exposed.
[0011] In one method for navigation on a bulk silicon device, after
a sample substrate has been sufficiently thinned by ion milling, it
is sometimes possible to visually differentiate highly doped wells
from the rest of the substrate in a FIB image. The outline of these
doped regions can be useful for navigational purposes. But during
backside milling on bulk silicon devices, it is easy to miss the
signal from the emerging doped-wells, which can lead to
over-milling and damage to the sample. The buried oxide surface
itself is very thin and fragile, and the signal from the buried
features is also weak and fleeting. Therefore an aggressive high
beam current and/or a long dwell time is required to distinguish
the transistor wells, which can even further damage the sample.
[0012] Real-time imaging using a separate electron beam is another
method for determining end-pointing. U.S. Pat. No. 7,388,218 to
Carleson for a "Subsurface Imaging Using an Electron Beam," which
is assigned to FEI Company of Hillsboro, Oreg., the assignee of the
present application, and which is incorporated herein by reference,
teaches an electron microscope that can image subsurface features.
The electron beam imaging concurrently with the ion beam allows
real time viewing of the milling process for end-pointing, and the
ability to view subsurface images gives a much greater margin of
error when exposing delicate buried features. Unfortunately, the
dual-beam system of Carleson suffers from a number of inherent
shortcomings. A dual-beam system is necessarily more complex and
expensive than a single beam system. Additionally, it is quite
difficult to keep both beams focused to the same focal point, which
also introduces error into the system. Although systems using
coincident and even coaxial ion and electron beams are known, such
systems are complex and still include a degree of inaccuracy that
it undesirable for many modern CE applications.
[0013] The use of helium ions for subsurface imaging is described
by Reiche et al. in "Applications of Helium Ion Microscopy in
Semiconductor Manufacturing," MICROSCOPY AND ANALYSIS, pp. 11-14
(July 2009). However, helium ions are not suitable for milling
applications because of the small size of the ions (and the
corresponding lack of physical sample damage that they cause). The
helium ion beam of Reiche would have to be combined with a separate
ion beam column using larger ions for any significant material
removal, and thus would suffer from the same disadvantages as
discussed above with respect to Carleson.
[0014] Thus, there is still a need for an improved method for
imaging and processing samples using FIB systems that allows both
for rapid, high accuracy navigation and end-pointing and for rapid
material removal once a feature has been located.
SUMMARY OF THE INVENTION
[0015] It is an object of the invention, therefore, to provide an
improved method and apparatus for imaging and milling a substrate
using a FIB system. Preferred embodiments of the present invention
use a mixture of light and heavy ions, focused to the same focal
point by the same beam optics, to simultaneously mill the sample
surface (primarily with the heavy ions) while the light ions
penetrate deeper into the sample to allow the generation of images
of subsurface features. Among other uses, preferred embodiments of
the present invention provide improved methods of navigation and
sample processing that can be used for various circuit edit
applications, such as backside circuit edit.
[0016] The foregoing has outlined rather broadly the features and
technical advantages of the present invention in order that the
detailed description of the invention that follows may be better
understood. Additional features and advantages of the invention
will be described hereinafter. It should be appreciated by those
skilled in the art that the conception and specific embodiments
disclosed may be readily utilized as a basis for modifying or
designing other structures for carrying out the same purposes of
the present invention. It should also be realized by those skilled
in the art that such equivalent constructions do not depart from
the spirit and scope of the invention as set forth in the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] For a more complete understanding of the present invention,
and the advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawings,
in which:
[0018] FIG. 1 shows a schematic representation of a prior art
backside IC device;
[0019] FIG. 2 shows a schematic representation of the backside IC
device of FIG. 1 after a wedge polish;
[0020] FIG. 3 is a graph showing Monte Carlo (SRIM) calculations of
the mean penetration depths of various ions in silicon;
[0021] FIGS. 4A and 4B show photomicrograph images of a backside IC
device after a wedge polish that has been imaged using relatively
high mass gallium ions;
[0022] FIGS. 5A and 5B show photomicrograph images of a backside IC
device after a wedge polish that has been imaged using relatively
low mass beryllium ions;
[0023] FIG. 6A shows a photomicrograph image of an area of an IC
device containing N-well contrast (doped silicon) regions that was
imaged using gold ions;
[0024] FIG. 6B shows a photomicrograph image of an area of an IC
device containing N-well contrast (doped silicon) regions that was
imaged using beryllium ions;
[0025] FIGS. 7A and 7B illustrate theoretical CRT line scans
showing signal versus position for two possible mixed ion
beams;
[0026] FIG. 8 is a graph showing the relationship between a mixed
beam suitable for pure imaging to a mixed beam suitable for pure
sputtering and the plasma gas ratio for an argon/xenon mixture;
[0027] FIG. 9 shows a graph of mass filter electromagnet voltage
versus time for two different possible duty cycles for switching
between light and heavy ions in a pseudo mixed beam; and
[0028] FIG. 10 shows a LMIS FIB system with a mass filter that
could be used to implement aspects of the present invention.
[0029] FIG. 11 shows the steps of an embodiment of the invention in
which etching and imaging are simultaneously performed.
[0030] The accompanying drawings are not intended to be drawn to
scale. In the drawings, each identical or nearly identical
component that is illustrated in various figures is represented by
a like numeral. For purposes of clarity, not every component may be
labeled in every drawing.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0031] Preferred embodiments of the present invention are directed
at methods of generating and using an ion beam composed of a
mixture of light and heavy ions to provide imaging of subsurface
features while still allowing for rapid material removal. In
preferred embodiments of the present invention, the light and heavy
ions are formed into a mixed beam so that they can be used
simultaneously to process a sample, with the heavy ions milling the
sample as in a conventional FIB system while the light ions
penetrate deeper into the sample to provide information about
subsurface features. In other preferred embodiments, a mass filter
can be used to rapidly switch between light and heavy ions at a
selected frequency to provide imaging of subsurface features while
milling, and to also allow adjustment of the ratio of light and
heavy ions produced by an alloy LMIS. A plasma FIB with multiple
gas sources can also be used to practice the present invention,
allowing the ratio of light and heavy ions to be controlled by
adjusting the composition of the gas used in the plasma ion
source.
[0032] A preferred method or apparatus of the present invention has
many novel aspects, and because the invention can be embodied in
different methods or apparatuses for different purposes, not every
aspect need be present in every embodiment. Moreover, many of the
aspects of the described embodiments may be separately
patentable.
[0033] FIB systems commonly used for circuit edit (CE) in
semiconductor manufacturing use ion beams formed from relatively
large ions, such as gallium, to mill away a substrate material to
expose hidden features. Sample imaging using such typical ion beam
systems is limited to the very top surface of the sample. As a
result, various techniques have been developed to navigate to and
safely uncover buried features for CE applications. Unfortunately,
these techniques are often time-consuming and require expensive
specialized equipment.
[0034] Preferred embodiments of the present invention overcome
these shortcomings of the prior art by simultaneously using a
mixture of light and heavy ions, focused to the same focal point by
the same beam optics, to simultaneously mill the sample surface
(primarily with the heavy ions) while the light ions penetrate
deeper into the sample to allow the generation of images of
subsurface features. Among other uses, preferred embodiments of the
present invention provide improved methods of navigation and sample
processing that can be used for various circuit edit applications,
such as backside circuit edit.
[0035] The penetration depth of ions in solids is dependant on the
mass of the ion, as illustrated by the graph in FIG. 3 showing
Monte Carlo (SRIM) calculations of the mean penetration depths of
various ions in silicon. While lighter ions such as helium or
beryllium are known to penetrate a substrate to a greater depth,
and thus to provide some degree of information on features
underneath the substrate surface, these lighter ions are much less
useful for milling applications. This is precisely because the
lighter ions tend to penetrate the sample surface rather than
impacting and sputtering away material like heavier ions such as
gallium.
[0036] Of course, the principal limitation for sub-surface imaging
is not necessarily the mean ion penetration depth, but rather the
depth from which secondary electrons can escape from the sample and
be detected. Nevertheless, metal layers on modern IC devices are
extremely thin (80-120 nm). Gallium ions have a mean penetration
depth of .about.27 nm in silicon, and therefore will not penetrate
deeper than one IC layer. However, ions with masses below .about.20
amu will do so at 50 keV, and ions below .about.12 amu will do so
at 30 keV. In this regard, He+, Li+, Be+, B+, O+, Ne+, and possibly
Si+ should be capable of penetrating typical IC layer
thicknesses.
[0037] As used herein, the terms "light ions" or "relatively low
mass ions" will be used to refer to ions that will penetrate at
least one IC layer. "Heavy ions" or "relatively high mass ions"
will refer to ions with greater masses than silicon (.about.28
amu), which are more suitable for rapid material removal. As used
herein, "rapid material removal" will refer to material removal
rates for a given sample type and beam configuration that are at
least as fast as the material removal rate for the sample type and
beam configuration using silicon ions. Persons of skill in the art
will recognize that the greater the amu difference between the two
ions species, the more subsurface information that will be provided
by the light ions species relative to the information provided by
the heavy ion imaging. Preferably the heavier elemental species
will have a mass that is at least double the mass of the lighter
elemental species. In some preferred embodiments, the heavier ions
will have a mass that is at least 40 amu greater than the lighter
ions; more preferably, the heavier ions will have a mass that is at
least 100 amu greater than the lighter ions.
[0038] Further, it should be noted that the term "secondary
electron" is typically used to refer to a free electron emitted
from a sample surface that is produced by the interaction of a
primary incident particle having sufficient energy with valence
electrons in the sample. Such emitted electrons with energies less
than 50 eV are called secondary electrons. Because of their low
energies, secondary electrons generated more that a certain
distance below the sample surface cannot escape from the specimen.
Although the maximum escape depth varies by compound, most
secondary electrons are produced within 2-5 nm of the surface.
Because light ions can be used to generate subsurface images at a
depth far greater than 5 nm, it does appear that there are other
mechanisms at work for ions penetrating deeply to change the number
of electrons escaping from the surface.
[0039] But in any case, features beneath the sample surface can be
imaged by detecting electrons escaping the substrate surface as a
beam of relatively light ions is scanned across the surface,
whether the escaping electrons are true secondary electrons,
backscattered ions, or result from charging effects, for example.
This type of subsurface imaging can be clearly seen by comparing
FIGS. 4A and 4B with FIGS. 5A and 5B. FIG. 4A shows a
photomicrograph image of a backside IC device after a wedge polish
that has been imaged using relatively high mass gallium ions. FIG.
4B is an enlargement of the area indicated by reference number 42
in FIG. 4A. This image of the IC device shows only vias (which
appear as the white dots 44 surrounded by the dark dielectric
material 46).
[0040] FIGS. 5A and 5B (where FIG. 5B is an enlargement of the area
indicated by reference number 52 shown in FIG. 5A) show similar
photomicrograph images of a backside IC device after a wedge polish
that has been imaged using relatively low mass beryllium ions. When
compared to FIGS. 4A and 4B, it is clear that a number of
sub-surface features, such as metal lines 54, can be clearly seen
in FIGS. 5A and 5B. These metal lines are buried underneath the
sample surface and are not visible at all in the surface images of
FIGS. 4A and 4B.
[0041] FIGS. 6A to 6B also illustrate the differences in images
produced by heavy and light ions for an area of an IC device
containing N-well contrast (doped silicon) regions. FIG. 6A was
imaged using relatively heavy gold (Au+) ions and almost no sample
features can be seen. FIG. 6B, however, was imaged using much
lighter beryllium (Be2+) ions. In this photomicrograph, features
beneath the surface can be clearly seen.
[0042] Any ion with a mass lighter than silicon should provide
superior sub-surface imaging, and any ion lighter than gallium will
be superior to what we can achieve today with conventional
gallium-based liquid metal ion sources (LMIS). According to
preferred embodiments, lighter mass ions will be able to penetrate
the sample surface to a depth of >80 nm, more preferably to a
depth of >120 nm. As a result, the preferred lighter ions will
also provide subsurface image information to a depth of >80 nm,
more preferably to a depth of >120 nm. Lighter ions will also
cause less sample damage, as compared to that caused by heavier
ions such as gallium. Of course, this means that lighter ions will
be much less efficient at removing material via sputtering, which
makes ion beams formed from light ions unsuitable for most circuit
edit (CE) applications.
[0043] Applicants have discovered, however, that because the
electrostatic optics used to focus an ion beam are insensitive to
ion mass, it is possible to focus an ion beam containing a mixture
of different size ions coaxially to the same focal point. In other
words, using a combined beam consisting of a heavy species, such as
gallium, and a light species, such as beryllium, the focused beam
position for both the heavy species and the light species will be
coincident. Using such a mixed beam, it is thus possible to
efficiently mill away material with the heavy ion component, while
using the lighter ions to obtain additional imaging information.
Because the light ions penetrate deeper into the sample, subsurface
features can actually be imaged directly while the milling is
taking place.
[0044] Preferred embodiments of the present invention provide
significant advantages in certain types of substrate processing,
for example in CE applications described above. Navigation to the
precise location of a buried target structure is much easier and
faster using the present invention because sub-surface structures
do not have to be exposed in order to use their coordinates to
locate a particular feature. Instead, using the sub-surface
information provided by the light ions, such features can be
located and correlated to the CAD design data. A convenient way of
doing this is to "overlay" the CAD shapes onto the secondary FIB
image and then to perform a two or three point CAD polygon
registration. Once the coordinates from the IC chip design have
been mapped to the actual sample, and the FIB system has navigated
to the approximate area of the target structure, any additional
sample registration is also easier and faster because local
features underneath the sample surface can also be observed and
used to re-register the sample and image. The present invention
also makes it much easier to expose the buried target structure
once it has been located. Because features can been seen in the
secondary FIB image before such features are exposed to the heavy
ions in the mixed beam, it becomes much easier to stop milling
before the target structure is damaged or destroyed.
[0045] Thus, in preferred embodiments of the present invention, an
alloy source capable of producing two different ions, one
relatively light (low mass) and one relatively heavy (high mass),
can be used to produce a mixed ion beam for imaging and processing
the sample. Preferably, the two different ions are not separated by
using a mass filter, but rather are both present in the beam that
is focused onto the sample. In some embodiments, a mass filter may
still be used to filter out other types of ions, for example when
the alloy source is a tertiary alloy source but it is only
desirable to use two of the ion species for imaging and processing
the sample. In other embodiments, as described below, a mass filter
can be used to rapidly switch between ions in order to "tune" the
ratio of light and heavy ions striking the sample surface.
[0046] Alloy sources are known in the prior art, and any such
source could be used to practice the present invention as long as
the source produces a suitable combination of light and heavy ions.
For example, AuSiBe, AuSi, and AsPdB alloy sources are commercially
available. As discussed in greater detail below, it will sometimes
be desirable to adjust the ratio of light to heavy ions for a
particular application. With an LMIS, the ratio is fixed by the
ratio of elements in the alloy. Most commercially available alloy
sources are eutectic compositions, which means that the percentages
of the elements result in the lowest melting point for the
combination. In addition to a lower melting point the bulk
composition remains more stable at the eutectic composition.
Although different compositions are typically possible, the
resulting melting point and vapor pressure of the resulting alloy
must be suitable for use as an alloy LMIS. A number of other
factors also contribute to the suitability of a particular alloy
for use as an alloy LMIS. These factors include: whether the
melting point is sufficiently low, the vapor pressure at the
melting point, whether the alloy reacts with typical (or easy to
use) substrate materials while at operating temperature, whether
the alloy wets the substrate material, whether the alloy is easy to
handle in air (for example, alloys containing Li and Cs are good
example of compositions that are not), and whether the bulk
composition remains constant over time/operation.
[0047] A suitable alloy LMIS for practicing the present invention
must also have a high enough content of a light ion to enable
adequate subsurface imaging of the desired features and a high
enough content of the heavy ion to allow material removal to
proceed at a high enough rate. FIGS. 7A and 7B illustrate
theoretical CRT line scans showing signal versus position for two
possible mixed ion beams. Although in actual mixed beam operation
it is difficult if not impossible to separate out the secondary
electron produced by the light ions from those produced by the
heavy ions, in these two signal graphs, lines 601 and 611
illustrate the electron signal that might be produced by the light
ion species, while lines 602 and 612 illustrate the electron signal
that might be produced by the heavy ion species. Lines 603 and 613
show the combined signal (respectively signal values shown by lines
601 plus 602 and lines 611 plus 612), which would be representative
of the signal that would be detected during actual operation.
[0048] In FIG. 7A, the mixed beam has a relatively low percentage
of light ions as compared to heavy ions. As the mixed ion beam is
scanned across the sample surface 604, the heavy ions do not
penetrate the featureless surface of the sample and so the changes
in signal reflect only system background noise. The lighter ions do
penetrate the sample surface and thus return information about
buried features 605 and 606. As shown in FIG. 7A, however, sample
605 is a relatively low contrast feature (harder to distinguish
from the surrounding substrate because of size, composition, depth,
etc.). While the signal line 601 does show the presence of buried
feature 605, the signal difference is small enough that it gets
lost in the much larger signal resulting from the heavy ion
species. As a result, in the combined signal line 603, it would be
very difficult to distinguish the contrast resulting from feature
605 from the signal noise. On the other hand, feature 606 is a
higher contrast feature, so even using a relatively low percentage
of light ions, the signal contrast is great enough that it can be
seen in the combined signal 603.
[0049] In FIG. 7B, the mixed beam has a higher percentage of light
ions as compared to heavy ions. As a result, even for low contrast
feature 605, the signal contrast is great enough that it can be
separated from the background noise in the combined signal. Of
course, the signal contrast for feature 606 would be even greater.
Significantly, the increased signal contrast for subsurface
features resulting from a greater percentage of light ions comes at
the price of decreased milling speed. For most applications, it
would be desirable to tune the relative percentages of the
light/heavy ions to achieve the minimum acceptable subsurface
imaging sensitivity (in other words, to achieve the minimum
acceptable percentage of light ions) in order to maximize milling
speeds. For example, high contrast subsurface images could be
identified in the combined signal even if the percentage of light
ions was relatively low. If the percentage of light ions gets too
low, however, the signal contrast from even high contrast features
would be lost in the much greater electron signal resulting from
the surface impact of the heavy ion species. For very low contrast
subsurface features, it might require a very high percentage of
light ions, in some cases approaching 100%.
[0050] In most cases, sufficient "tuning" of the percentages of
light and heavy ions can be accomplished by through the selection
of a particular standard alloy LMIS source. Persons of skill in the
art will be able to balance the considerations for a particular
application to select a suitable known alloy LMIS for most
applications without undue experimentation.
[0051] In other cases, however, it may be desirable to more finely
tune the relative percentages or to adjust the percentages
"on-the-fly" as a sample is being processed. U.S. patent
application Ser. No. 12/373,676 by Smith, et al., for "Multi-Source
Plasma Focused Ion Beam System," which is assigned to the Assignee
of the present invention and is hereby incorporated by reference,
describes a plasma FIB that is capable of switching between
multiple gas sources. A similar apparatus, as described in greater
detail below, could be used to deliver multiple gases at the same
time, resulting in a mixed beam containing multiple ion
species.
[0052] Using such a multi-source plasma FIB, the relative
percentages of the light and heavy ions could be easily adjusted to
any desired percentage, for example between pure imaging (100%
light ions) and pure sputtering (100% heavy ions) as shown in FIG.
8 for a mixture of gold and xenon ions. The use of such a
multi-source plasma FIB would also allow the percentages to be
adjusted "on-the-fly" so that, for example, a greater concentration
of light ions could be used for imaging subsurface images for
navigation purposes, but the percentage of heavy ions could be
increased for faster milling once the buried feature of interest is
located. Because the mix of ion species would be controlled via the
gas sources, preferred embodiments of the present invention could
be practiced using a plasma FIB without a mass filter.
[0053] In the case of an alloy LMIS, the composition of the source
itself cannot be adjusted, although it would be possible to produce
customized sources that would produce desired percentages of light
and heavy ions. Significantly, when using an alloy LMIS containing
an appropriate percentage of light and heavy ions, a mass filter
would also not be necessary, which would greatly decrease system
cost and complexity.
[0054] Although the composition of the alloy source cannot be
adjusted "on-the-fly" like the gas composition in a multi-source
plasma FIB, the use of an alloy LMIS system that is equipped with a
mass filter would allow for some adjustment of the relative
percentages of light and heavy ions impacting the surface.
According to preferred embodiments of the present invention, a mass
filter could be used to rapidly switch between ion species at a
selected frequency during material processing. For example, the
mass filter could allow only light ions to strike the sample for a
short period, then switch to heavy ions for another set very short
period. By rapidly alternating between light ions and heavy ions at
a set frequency, the sample imaging can be made to appear to an
operator simultaneous with the sample milling. Although the use of
light and heavy ions would be alternating rather than simultaneous,
it would appear to an operator as though a true mixed and
coincidental beam were being used.
[0055] The rate at which a given system can switch between light
and heavy ions to produce such a "pseudo" mixed beam will depend
upon the how frequently the magnetic mass filter can be adjusted. A
typical mass filter can be adjusted between ion species by applying
a periodic function to the mass filter voltage with a frequency in
the MHz range, although the time required for the magnet to settle
would preferably be built into the data acquisition so that data
(imaging) is only collected at appropriate times. A duty cycle
switching between ion species of approximately a tenth of a second
would be fast enough that it would not visually disturb an operator
viewing the sample processing in real time. By adjusting the length
of time that each ion species is allowed through the mass filter,
the beam could be adjusted to favor non-destructive imaging or
aggressive sputtering, depending on the application.
[0056] This is illustrated in FIG. 9, which shows a graph of mass
filter electromagnet current versus time for two different possible
duty cycles for switching between light and heavy ions in a pseudo
mixed beam. Dashed line 900 and solid line 902 each show the
voltage applied to the mass filter magnet over time, with a
periodic function applied that switches the magnet between V.sub.1,
where only the heavy ion species is allowed through the mass
filter, and V.sub.2, where only the light ion species is allowed
through. As shown in the graph, for the duty cycle shown by dashed
line 900, the mass filter stays at voltage M2 (allowing light ions
through) approximately five times longer than in the cycle shown by
solid line 902. As a result, sample processing using the cycle
shown by line 900 would result in a relatively higher percentage of
light ions striking the sample per cycle than would the cycle shown
by line 902. This would make the cycle of line 900 more suitable
for less destructive imaging, while the cycle of line 902 would
result in a greater number of large ions and thus more rapid
material removal. The frequency for both duty cycle times in FIG. 9
is approximately 10 Hz, although different frequencies could be
used.
[0057] FIG. 10 shows a typical FIB system 210 that could be used to
implement preferred embodiments of the present invention. The
present invention could also be implemented using other particle
beam systems, including for example, dual-beam systems, such as
FIB/SEM dual beam system.
[0058] Focused ion beam system 210 includes an evacuated envelope
211 having an upper neck portion 212 within which are located an
ion source 214 and a focusing column 216 including extractor
electrodes 215 and an electrostatic optical system including
condenser lens 217 and objective lens 252. Ion source 214 is
preferably an alloy LMIS which produces ions of more than one
elemental species, preferably a combination of light and heavy
ions. As used herein, the phrase "different elemental species" (or
"more than one elemental species") is used to refer to ions having
a different elemental composition. Typically ions will be of at
least two different elements entirely, such as the mixed beam of
gold and beryllium ions described above. In some embodiments,
however, one or both of the mixed ions could be an ion composed of
more than one element (AuBe+, for example). Once the ions, both
light and heavy, are extracted from the source, they are
accelerated and focused onto the sample by way of electrostatic
lenses within focusing column 216. In other preferred embodiments,
a plasma source could be used, preferably one including multiple
gas sources.
[0059] Ion beam 218 passes from ion source 214 through column 216
and between electrostatic deflection means schematically indicated
at 220 toward sample 222, which comprises, for example, a
semiconductor device positioned on movable X-Y-Z stage 224 within
lower chamber 226. An ion pump or other pumping system (not shown)
can be employed to evacuate neck portion 212. The chamber 226 is
evacuated with turbomolecular and mechanical pumping system 230
under the control of vacuum controller 232. The vacuum system
provides within chamber 226 a vacuum of between approximately
1.times.10-7 Torr and 5.times.10-4 Torr. If an etch assisting, an
etch retarding gas, or a deposition precursor gas is used, the
chamber background pressure may rise, typically to about
1.times.10-5 Torr.
[0060] High voltage power supply 234 is connected to ion source 214
as well as to appropriate electrodes in focusing column 216 for
forming an ion beam 218 and directing the same downwardly.
Deflection controller and amplifier 236, operated in accordance
with a prescribed pattern provided by pattern generator 238, is
coupled to deflection plates 220 whereby beam 218 may be controlled
to trace out a corresponding pattern on the upper surface of sample
222. In some systems the deflection plates are placed before the
final lens, as is well known in the art.
[0061] The ion source 214 typically provides a metal ion beam of
gallium, although other ion sources, such as a multicusp or other
plasma ion source, can be used. The ion source 214 typically is
capable of being focused into a sub one-tenth micron wide beam at
sample 222 for either modifying the sample 222 by ion milling,
enhanced etch, material deposition, or for the purpose of imaging
the sample 222. When the ions in the ion beam 218 strike the
surface of work piece 222, secondary electrons and backscattered
electrons are emitted. A charged particle multiplier 240 used for
detecting secondary ion or electron emission for imaging is
connected to signal processor 242, where the signal from charged
particle multiplier 240 are amplified, converted into digital
signals, and subjected to signal processing. The resulting digital
signal is to display an image of sample 222 on the monitor 244.
[0062] A door 270 is opened for inserting sample 222 onto stage
224, which may be heated or cooled, and also for servicing an
internal gas supply reservoir, if one is used. The door is
interlocked so that it cannot be opened if the system is under
vacuum. The high voltage power supply provides an appropriate
acceleration voltage to electrodes in ion beam column 216 for
energizing and focusing ion beam 218.
[0063] A gas delivery system 246 extends into lower chamber 226 for
introducing and directing a gaseous vapor toward sample 222. U.S.
Pat. No. 5,851,413 to Casella et al. for "Gas Delivery Systems for
Particle Beam Processing," assigned to the assignee of the present
invention, describes a suitable gas delivery system 246. Another
gas delivery system is described in U.S. Pat. No. 5,435,850 to
Rasmussen for a "Gas Injection System," also assigned to the
assignee of the present invention. For example, iodine can be
delivered to enhance etching, or a metal organic compound can be
delivered to deposit a metal.
[0064] System controller 219 controls the operations of the various
parts of dual beam system 20. Through system controller 219, a user
can cause ion beam 218 to be scanned in a desired manner through
commands entered into a conventional user interface (not shown).
System controller 219 can also comprise computer-readable memory
221 and may control dual beam system 210 in accordance with data or
programmed instructions stored in memory 221. CAD data concerning
the sample/semiconductor stored in memory 221 can be used to create
a CAD polygon overlay or other positional data used to locate a
feature of interest and alignment points or transfer fiducials as
described above.
[0065] Optionally, FIB system 210 can also include a mass separator
such as mass filter 250 to separate out a single ion species from
the combination of heavy and light species provided by an alloy
LMIS or plasma source. When a magnetic field is applied by mass
filter 250, the mixed ion beam will be spread out by mass. A proper
selection of voltage will allow only one ion species to pass
through the mass selection aperture 251 and on through the lower
column to the sample. A different voltage will allow the other ion
species to pass through the aperture. Preferably, mass filter 250
will be capable of rapidly switching between selected voltages to
alternate light and heavy ion beams with a frequency in the MHz
range.
[0066] FIG. 11 shows the steps of an embodiment of the invention in
which etching and imaging are simultaneously performed. Step 1102
includes providing an ion source that produces at least two species
of ions. Step 1104 includes providing an ion beam column for
producing a beam of ions, the beam including the at least two
species of ions, each having different a elemental composition,
with one ion species being having a lower mass and one ion species
having a higher mass. Step 1106 includes directing the particle
beam including at least two species of ions toward the sample to
process and image the sample simultaneously. Step 1108 then
includes detecting the secondary electrons emitted from the sample
resulting from the impact of the lighter ions to form an image of
at least one subsurface feature. In step 110, if the adequate
features have been exposed, using the location of the imaged
subsurface features to direct the ion beam toward the sample. In
step 1111 the sample is processed using the ions of the heavier ion
species to mill the sample surface. The method ends with step 1112
as the processing and imaging is complete. In some embodiments, the
percentage of particles in the beam can be altered to fine-tune the
imaging and processing capabilities of the focused ion beam. A
higher concentration of heavy ions will allow material to be
removed at a high rate, while a higher concentration of light ions
will allow adequate subsurface imaging.
[0067] Although the description of the present invention above is
mainly directed at methods of generating and using an ion beam
composed of a mixture of light and heavy ions, it should be
recognized that an apparatus performing the operation of such a
method would further be within the scope of the present invention.
Further, it should be recognized that embodiments of the present
invention can be implemented via computer hardware or software, or
a combination of both. The methods can be implemented in computer
programs using standard programming techniques--including a
computer-readable storage medium configured with a computer
program, where the storage medium so configured causes a computer
to operate in a specific and predefined manner--according to the
methods and figures described in this Specification. Each program
may be implemented in a high level procedural or object oriented
programming language to communicate with a computer system.
However, the programs can be implemented in assembly or machine
language, if desired. In any case, the language can be a compiled
or interpreted language. Moreover, the program can run on dedicated
integrated circuits programmed for that purpose.
[0068] Further, methodologies may be implemented in any type of
computing platform, including but not limited to, personal
computers, mini-computers, main-frames, workstations, networked or
distributed computing environments, computer platforms separate,
integral to, or in communication with charged particle tools or
other imaging devices, and the like. Aspects of the present
invention may be implemented in machine readable code stored on a
storage medium or device, whether removable or integral to the
computing platform, such as a hard disc, optical read and/or write
storage mediums, RAM, ROM, and the like, so that it is readable by
a programmable computer, for configuring and operating the computer
when the storage media or device is read by the computer to perform
the procedures described herein. Moreover, machine-readable code,
or portions thereof, may be transmitted over a wired or wireless
network. The invention described herein includes these and other
various types of computer-readable storage media when such media
contain instructions or programs for implementing the steps
described above in conjunction with a microprocessor or other data
processor. The invention also includes the computer itself when
programmed according to the methods and techniques described
herein.
[0069] Computer programs can be applied to input data to perform
the functions described herein and thereby transform the input data
to generate output data. The output information is applied to one
or more output devices such as a display monitor. In preferred
embodiments of the present invention, the transformed data
represents physical and tangible objects, including producing a
particular visual depiction of the physical and tangible objects on
a display.
[0070] Preferred embodiments of the present invention also make use
of a particle beam apparatus, such as a FIB or SEM, in order to
image a sample using a beam of particles. Such particles used to
image a sample inherently interact with the sample resulting in
some degree of physical transformation. Further, throughout the
present specification, discussions utilizing terms such as
"calculating," "determining," "measuring," "generating,"
"detecting," "forming," "superimposing," "imaging," "navigating" or
the like, also refer to the action and processes of a computer
system, or similar electronic device, that manipulates and
transforms data represented as physical quantities within the
computer system into other data similarly represented as physical
quantities within the computer system or other information storage,
transmission or display devices or that controls the operation of a
particle beam system.
[0071] The invention has broad applicability and can provide many
benefits as described and shown in the examples above. The
embodiments will vary greatly depending upon the specific
application, and not every embodiment will provide all of the
benefits and meet all of the objectives that are achievable by the
invention. Particle beam systems suitable for carrying out the
present invention are commercially available, for example, from FEI
Company, the assignee of the present application.
[0072] Although much of the previous description is directed at
semiconductor wafers, the invention could be applied to any
suitable substrate or surface. Further, whenever the terms
"automatic," "automated," or similar terms are used herein, those
terms will be understood to include manual initiation of the
automatic or automated process or step. In the following discussion
and in the claims, the terms "including" and "comprising" are used
in an open-ended fashion, and thus should be interpreted to mean
"including, but not limited to . . . . " The term "integrated
circuit" refers to a set of electronic components and their
interconnections (internal electrical circuit elements,
collectively) that are patterned on the surface of a microchip. The
term "semiconductor device" refers generically to an integrated
circuit (IC), which may be integral to a semiconductor wafer,
singulated from a wafer, or packaged for use on a circuit board.
The term "FIB" or "focused ion beam" is used herein to refer to any
collimated ion beam, including a beam focused by ion optics and
shaped ion beams.
[0073] To the extent that any term is not specially defined in this
specification, the intent is that the term is to be given its plain
and ordinary meaning. The accompanying drawings are intended to aid
in understanding the present invention and, unless otherwise
indicated, are not drawn to scale.
[0074] Although the present invention and its advantages have been
described in detail, it should be understood that various changes,
substitutions and alterations can be made to the embodiments
described herein without departing from the spirit and scope of the
invention as defined by the appended claims. Moreover, the scope of
the present application is not intended to be limited to the
particular embodiments of the process, machine, manufacture,
composition of matter, means, methods and steps described in the
specification. As one of ordinary skill in the art will readily
appreciate from the disclosure of the present invention, processes,
machines, manufacture, compositions of matter, means, methods, or
steps, presently existing or later to be developed that perform
substantially the same function or achieve substantially the same
result as the corresponding embodiments described herein may be
utilized according to the present invention. Accordingly, the
appended claims are intended to include within their scope such
processes, machines, manufacture, compositions of matter, means,
methods, or steps.
* * * * *